The role of cholesterol and cytochrome P-450 in the cholesterol side chain cleavage reaction in adrenal cortex and corpora lutea

Abstract

1. 1. Rat adrenal mitochondria, and rat corpora luteal mitochondria in vitro convert endogenous cholesterol into pregnenolone by a cholesterol side chain cleavage mixed function oxidase. 2. 2. These mitochondria contain a relatively small pool of cholesterol so that the decline in the observed rate of pregnenolone production over a few minutes at 37°C is due to substrate depletion. 3. 3. It is possible to maintain the initial rate of pregnenolone formation by the inclusion of cholesterol in the incubation medium added in a small quantity of acetone or cholesterol offered in the form of a lecithin-cholesterol micelle. 4. 4. The cytosol of the rat adrenal cortex and the cytosol of the rat corpora lutea both contain cholesterol ester hydrolases which are activated by a protein kinase in the presence of 3'5'-cyclic AMP and ATP. This activation of the cholesterol ester hydrolase is independent of protein synthesis blockers such as cycloheximide. 5. 5. It is suggested that one method by which the trophic hormones ACTH and LH may activate their appropriate target tissues is by a 3'5' cyclic AMP dependent activation of a protein kinase resulting in a phosphorylation and activation of the cholesterol esterase. 6. 6. This esterase can attack the cholesterol esters in the lipid droplets bringing about a release of unesterifled cholesterol. 7. 7. The cholesterol which is in the cytoplasm moves to the mitochondria by a mechanism which is not yet clear. After the initial rapid uptake by the mitochondria, the cholesterol has to move throughout the cristae to the active site of the cholesterol side chain cleavage enzyme system. 8. 8. The evidence suggests that the translocation or the transportation of the sterol molecule within the mitochondrion may be the rate limiting event in the overall cholesterol side chain cleavage oxygenase reaction. Labile protein(s) may be involved in this event and also in other aspects of the cholesterol side chain cleavage reaction. 9. 9. From studies on intact mitochondria and on the isolated cytochrome P-450 involved in the cholesterol side chain cleavage reaction it seems clear that the micro-environment of the cholesterol (?phospholipid) cytochrome P-450 in the cristae may be a significant factor in the ultimate rate of the cholesterol side chain cleavage reaction.